Foodstuffs, drinks, drugs, and the like are required to be stored in the absence of oxygen or under an extremely low oxygen condition because deterioration in the quality is caused due to oxygen.
For that reason, a container or a package for storing a foodstuff, a drink, a drug, or the like is filled with nitrogen; however, this method has problems, for example, that manufacturing costs increase and that when the container or the package is opened once, air flows thereinto from the outside, and thereafter, deterioration in the quality cannot be prevented. Accordingly, various investigations have been made for the purpose of absorbing residual oxygen in the container or the package to remove oxygen from the inside of the system.
As a method for removing oxygen in a container or a package, a method in which a separate small bag accommodating therein an oxygen absorbent mainly composed of an iron powder is disposed has hitherto been widely employed. However, though this method keeps the cost low and attains a fast oxygen absorption rate, it causes inconvenience in the case where a metal detector is used for the purpose of detecting a foreign substance or in the case where the container or the package is applied for a microwave oven as it is.
In the case of a resin-made container or packaging material, investigations have been made for imparting oxygen absorbability to the container or packaging material per se.
For example, it is proposed to use an oxygen absorbent comprising a polyterpene such as poly(α-pinene), poly(β-pinene) or poly(dipentene) and a transition metal salt acting as an oxygen absorption catalyst such as cobalt neodecanoate or cobalt oleate (Patent Document 1).
It is also proposed to use an oxygen absorbent comprising a conjugated diene polymer such as polyisoprene or 1,2-polybutadiene and a transition metal salt (Patent Document 2).
Furthermore, it is proposed to use an oxygen absorbent comprising a copolymer of ethylene with cyclopentene and a transition metal salt (Patent Document 3).
However, each of the compositions disclosed in these patent documents does not exhibit sufficient oxygen absorption performances and involves a problem that a peculiar odor, which is presumably caused by-products of oxygen-trapping reactions, remains in a packaging container. Further, since all of them contain a transition metal, they have risks that the polymer is liable to be deteriorated with the progress of oxygen absorption reactions causing remarkable reduction in mechanical strengths of the packaging material and that the transition metal salt is liable to elute. Therefore, it is difficult to employ them in some applications.
Patent Document 1: WO 98/06779
Patent Document 2: JP-A-2003-71992
Patent Document 3: WO 01/003521